DESCRIPTION (Adapted from Applicant s Description): Background: Although many lines of evidence document the importance of maintaining a relatively high calcium intake during pubertal development, the calcium intake of as many as 20% of pubertal American girls is less than one-third of the currently recommended level. Virtually no information is available regarding the physiological adaptation to these low intakes. For example, despite a lower average calcium intake, peak bone mass is greater and the risk of osteoporosis is lower in African-American compared with Caucasian girls. This proposal will evaluate the adaptation to low calcium intakes by girls during puberty and compare the differences in that adaptation between Caucasian and African-American girls. Methods: Studies will be performed in two groups of healthy premenarcheal girls, ages 9-13 years, and Tanner Stages 2 and 3. Each group will consist of 15 Caucasians and 15 African-Americans. Group 1 will consist of those girls with habitual calcium intakes of 700 to 1300 mg/day. Group 2 will consist of girls with habitual calcium intakes of 300 to 500 mg/day. Baseline studies of calcium absorption, urinary and endogenous fecal calcium excretion and calcium kinetics will be performed in girls in both groups. Girls in Group 1 then will have their dietary calcium intake decreased to 400 mg/day for 21 days, and calcium studies will be repeated. Girls in Group 2 will take 600 mg calcium supplements (as calcium citrate malate) for 6 months prior to repeating the study. Calcium absorption/excretion and kinetic measurements will be performed in all subjects using a dual-tracer stable isotope technique with complete urine and fecal collections for 6 days after isotope administration. In addition to the groups of healthy girls, a group of 8 girls with Turner's syndrome will participate in calcium studies on a diet containing 400 mg/day of calcium before and after 3 months of estradiol therapy. Interpretation: This study will provide a systematic evaluation of the physiological adaptations to low calcium intakes by African-American and Caucasian girls during puberty and of the relationships among pubertal ethnicity, pubertal hormones, and the adaptive response to low calcium intakes. By studying girls with Turner's syndrome, the investigators will further evaluate the relationship between estrogen status and calcium metabolism. These studies will provide novel information which will be useful in providing guidance to the large proportion of the young adolescent population with very low calcium intakes.